Automatic postural adjustments responsible for human balance will be studied in normal adults and in patients with disorders of posture control. The goal of these studies is to define, quantitatively, the normal repertoire of postural movement strategies in standing humans for different speeds and directions, to address the question of relative contribution of various types of sensory feedback versus central programming in determination of postural strategy selection and then to apply this approach to differentiating between the postural ataxia associated with specific types of peripheral and central CNS-disordered patients. The long term aim is to improve assessment and treatment of patients with postural instability. Subjects will stand on a movable platform system. Postural adjustments to a variety of different movement perturbations will be described by analyzing the EMG activity of selected leg and lower trunk muscles and by recording surface forces and motions of the ankle, knee, hip and neck angles. Experiments are designed to test the hypothesis that the CNS preprograms a discrete set of automatic postural strategies, each of which is normally triggered within bounded stimulus conditions and that these boundaries can change with neural pathology causing postural instability. Select groups of patients with peripheral vestibular deficits, peripheral neuropathy and cerebellar deficits will be studied. The ability of patients to select postural strategies appropriate to conditions as well as the intactness of their postural synergy patterns will be determined.